The subject matter disclosed herein relates to electric machines and, specifically, permanent magnet (PM) machines.
PM machines are used in various applications (e.g., aviation, propulsion motor for passenger vehicles, military ground vehicles, etc.) to convert between electrical power and mechanical power. Conventional PM synchronous electric machines employ permanent magnets as the magnetic poles of a rotor, around which a stator is disposed. The stator has a plurality of teeth that face the rotor. Alternatively, the machine may be designed so that the rotor surrounds the stator. For high-speed operation, a retaining sleeve is usually wrapped around the magnets as needed to keep the magnets in place. The retaining sleeve may be shrink fit upon the magnets to ensure a non-slip fit. Usually the retaining sleeve is made of one whole metallic piece for structural integrity. When the coils formed on the stator are energized, a magnetic flux is induced by the voltage, creating electromagnetic forces between the stator and the rotor. These electromagnetic forces contain tangential and/or circumferential forces that cause the rotor to rotate. When a PM machine is operating in the generating mode and experiences a fault (e.g., a short circuit due to winding defects or defective components), it may not be possible to quickly stop the PM machine because it is externally driven by the mechanical system. A fault-tolerant PM machine may be capable of sustaining a fault condition indefinitely. However, typical approaches to increasing fault tolerance may negatively impact the torque density of the PM machine. As such, it would be beneficial to improve the fault tolerance of a PM machine without sacrificing torque density.